The field of data communications typically uses a modem to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems to communicate large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multidimensional signal space constellation. In some instances, a onedimensional signal space constellation can be employed, such as in the case of pulse amplitude modulation (PAM). The constellation can include both analog and digital information or only digital information.
In the above mentioned communications system, typically both digital data and an analog signal are to be transmitted. The data signal to be transmitted is represented by a sequence of data symbols, where each data symbol is associated with a particular N-dimensional signal point value taken from a signal space. Similarly, the analog signal, which for example can be represented by a voice signal, is processed so that it is mapped into the N-dimensional signal space to provide a voice signal point. This voice signal point defines the magnitude and angle of a voice signal vector about the origin of the signal space. The data symbol and the voice vector are then added together to select a resultant N-dimensional signal point. These N-dimensional signal points are grouped into either square or circular signal space constellations and then transmitted to a far-end modem.
Upon reception of the transmitted N-dimensional signal point, the receiver of the far-end modem detects the embedded data symbol and subtracts the data symbol from the received N-dimensional signal point to yield the voice signal vector. This voice signal vector is then used to recreate the voice signal.
Because square constellations have a higher peak factor, square constellations typically require more power to transmit a given amount of information than circular constellations. Square constellations also have a greater susceptibility to harmonic distortion. For example, in a 256 point two dimensional circular constellation, the constellation point, or symbol, with the highest power has the x, y coordinates 17, 5 with a peak power of 17.sup.2 +5.sup.2 =314, whereas the highest power point in a square constellation has the x, y coordinates 15, 15 with a peak power Of 15.sup.2 +15.sup.2 =450. As can be seen, the peak power of a circular constellation is 1.6 dB lower than that of a square constellation.
The modulation technique used to transmit the signal space constellation can be either coded or uncoded. Coded modulation entails encoding the data signal prior to transmission and decoding the received coded data signal in a receiver. Coded modulation such as coded quadrature amplitude modulation (QAM) as known in the art typically includes various forward error correction (FEC) techniques, such as block coding, convolutional coding and trellis coding. The FEC code acts on a discrete data channel whereby an encoder maps the source data to q-ary code symbols which are modulated and transmitted. During transmission, this signal can be corrupted, causing errors to arise in the demodulated symbol sequence. An FEC decoder attempts to correct these errors and restore the original source data. Trellis coding is presently considered state of the art for coded modulation. Coded modulation, however, consumes power and available bandwidth by requiring additional computational cycles by the system processor to provide the forward error correction code.
Previously, circular constellations have been implemented using coded modulation in which trellis coding is employed to reduce the error rate, and square constellations have been implemented using CAP modulation, which uses Tomlinson preceding to reduce the error rate. Transmitting a square constellation increases the peak factor of the transmitted signal, thus causing an increase in power consumption and harmonic distortion. Therefore, it would be beneficial to develop a way in which to transmit a circular signal space constellation in an uncoded modulation environment to maximize processor resources and improve efficiency.